This invention relates to semiconductor die mounting means and more particularly to such die that are mounted to an electrically conductive substrate and dielectrically isolated therefrom.
Various methods for attachment and mounting of semiconductor die are performed, based on device circuitry to lead frame electrical and thermal relationships. Discrete high power devices require excellent thermal as well as electrical conduction. One method of attachment comprises sputtering gold onto the die mounting face and bonding the die to the metal substrate (lead frame) by soldering or by means of a conductive resin, e.g. a silver loaded epoxy resin may be disposed between the two surfaces. Integrated medium or low power semiconductor circuits that do not require either electrical or thermal conduction to the die back side are typically attached by a non-conductive epoxy between the two bonding surfaces.
It is also known to bond the back face of a semiconductor die to a substrate by means of a dielectric material, e.g. epoxy resin, polyimide, and the like. In such cases, the substrate is generally not conductive but is itself a dielectric, e.g. an alumina substrate. On the other hand, there are instances wherein a high thermal but dielectric connection is made between one face of a high-power handling semiconductor die and a metal heat sink. This is often accomplished by growing a non-conducting oxide on the die face to be thermally contacted and either bonding the oxide carrying face by a resin to the heat sink or spring loading the die face against the heat sink usually with a thin layer of a grease therebetween.
None of the above mentioned prior art die-mounting means are suitable for those situations wherein a high dielectric breakdown voltage must be maintained between the die mounting face and a conducting substrate. The silicon dioxide and grease layers are too thin, i.e. cannot be relied upon to withstand voltages greater than about 200 volts. Also, the complicating spring-loaded feature is costly. A simple dielectric-resin bonding means can be used. To do so requires unusual care to dispense the desired amount of liquid resin to the substrate and then to place the die in the liquid resin so that there is everywhere a minimum thickness of the resin between die and substrate to guarantee the desired high voltage capability after the resin is cured. In general, that process is critical, costly and leads to low yields.
It is an object of the present invention to provide an economical, manufacturable and reliable assembly of a semiconductor die dielectrically mounted to a substrate capable of withstanding high voltages therebetween.